This invention relates to a process and apparatus useful in the treatment of slurries of cellulosic material. In general, it has applicability to any such treatment wherein it becomes desirable to increase the consistency of the slurry, as, for example, by filtration, and wherein some of the cellulosic material "breaks through" the filtering medium. It is particularly applicable when the slurry is filtered on a horizontal belt type filtering apparatus and the filter cake then contacted with a treatment liquid, while still on the horizontal filter medium, which displaces at least a portion of the liquid carried by the filter cake. Thus, for example, the invention finds utility in the recovery of the liquor associated with the digester stock in a conventional pulp producing operation, such liquor often referred to by those skilled in the art as the "black liquor" or "digester liquor". The digester stock is normally a relatively non-flowable slurry of pulp and digester liquor which must be diluted to a flowable consistency before it can be further processed. As is known in the art, recovery of the digester liquor generally involves one or more filtrations of the diluted digester stock to remove the digester liquor from the pulp solids, the objective being to recover as much of the liquor as possible for regeneration of the pulping chemicals, and to provide as liquor-free or clean a pulp as possible for further processing.
At the present time, probably the most common commercial procedure for recovering digester liquor involves filtering the diluted digester slurry on a rotary drum vacuum filter. Since one filtration ordinarily does not suffice the filter cake is typically reslurried with wash water and then filtered again on the rotary filter. This sequence is repeated as many times as required, usually in counter-current fashion, to obtain the desired recovery of digester liquor. The recovered digester liquor is then further processed in accordance with conventional procedures.
A major disadvantage of the drum filters is that the recovered digester liquor is usually significantly diluted as the result of being mixed with water during the reslurrying operation. This dilution effect is further aggravated by the fact that drum filters require very low slurry consistencies for most efficient operation and hence substantial amounts of water are required in the reslurrying of the filter cake. Because the pulp and digester liquor retained thereby is contacted with copious amounts of wash liquid during the reslurrying step, the pulp solids are eventually washed free of the digester liquor by predominantly a dilution effect.
A highly diluted digester liquor is a process disadvantage because the normal procedure in many pulping processes is to burn the digester liquor to remove the volatiles and recover the non-volatile components for eventual recycle to the digester system. Unfortunately, however, the liquor cannot be effectively burned if unduly diluted, and the common practice, therefore, is to subject the recovered digester liquor to an energy consuming evaporation-concentration step prior to burning it to increase its concentration to a burnable level. In short, a good portion of the water deliberately added in the liquor recovery operation must now be removed. It is apparent, therefore, that a process for recovering the digester liquor in a highly undiluted state would be most advantageous.
A number of solutions have been proposed to overcome the dilution problem. One such solution involves the replacement of the drum filtration apparatus by a horizontal belt filtering apparatus wherein the diluted digester slurry is deposited on a flat advancing horizontal filter medium operating under vacuum. The digester liquor is sucked from the slurry leaving a mat of pulp on the filter medium. Wash water is then applied to the mat surface. As the wash water enters the mat, it "displaces" the digester liquor therein by what is believed to be essentially a plug-flow mechanism in which very little dilution of the displaced liquid by the treatment liquid takes place. As a result, the mat can be washed in this fashion to recover the digester liquor contained therein in a relatively undiluted condition. The mat is normally washed counter-currently while it remains on the flat advancing filter medium. On the general concept of recovering digester liquor in a substantially undiluted condition by filtering the pulp slurry and then counter-currently washing the pulp mat on a horizontal belt filter, see the work of G. Tombler et al. reported in ENVIRONMENT CANADA Project Report 82-1 "Comparison of Brown Stock Washing Systems" (July, 1973), available from CPAR Secretariat, Canadian Forestry Service, Dept. of the Environment, Ottawa, Canada, KlA OH3, said publication incorporated herein by reference.
It has been found, however, that despite the advantages in using a horizontal belt filter, there are a number of problems associated with such a process which detract from its usefulness. One major disadvantage is the break-through of pulp through the filter medium, especially during the initial formation of the pulp mat. The practical efficient usage of a horizontal belt filter requires that the filter medium be (1) highly durable, (2) resist plugging, and (3) provide an acceptable drainage rate. For these reasons, it is the common practice in the trade to fabricate the filtering medium from a weave of rather coarse but strong filaments, e.g., of diameter of 10 mils or greater. This, coupled with the need for a relatively high percentage open area to provide the desired drainage rate and plug resistance required, results in the filter medium passing not only the liquid phase of the cellulosic slurry deposited thereon but also some of the pulp itself. Pulp "break-through" normally continues until an appreciable pulp mat builds up on the filtering medium. It has been found, for example, that anywhere from about 0.1 to 1.5% of the pulp may "break through" and be lost if some sort of recovery system is not provided. At a present pulp value of about $300/ton, such losses would not be acceptable.
Another problem encountered with the use of a horizontal belt filtering apparatus is the foaming of the various digester liquor streams separate from the pulp. This foaming arises from a combination of several factors. First, the digester liquor is under vacuum since vacuum is used to assist the filtration and/or washing operation. Secondly, the digester liquor is relatively hot, e.g., on the order of about 200.degree. F. and thus near its atmospheric boiling point. Thirdly, the digester liquor includes surface active agents. Finally, breaks in the filter vacuum can occur from time to time because of poor sealing, causing temporary injections or surges of air into the separated digester liquor. The foaming which results can be quite serious and extensive. The problem is that the foam becomes entrained in the gas streams of the vacuum system and is lost to the process through the vacuum system exhaust. In addition to making day-to-day operation more difficult, a more serious problem of the foaming is the loss of digester liquor it can cause. For example, anywhere from about 2 to 13% or more of the digester liquor can be lost through foaming, adversely affecting the economics of the process.
The loss of vacuum referred to above usually occurs because of the difficulty of adequately sealing the edges of the filtering medium and its supporting filter belt. The thickness of the filaments making up the filter medium further aggravates this problem by preventing an air-tight seal at the edges.
Another problem of the use of horizontal belt filters is the difficulty of establishing or re-establishing equilibrium or steady-state operation, particularly in the counter-current washing operation, either during start-up or once an upset has been introduced into the system. These difficulties make routine, efficient operation more difficult.
It is toward the solution of these problems that the present invention is directed, and toward the provision of a technique for effectively utilizing the advantageous low dilution characteristics of the horizontal belt filtering and washing system, but without the previously encountered drawbacks of the system.
It will be apparent, however, that the features provided by the present invention are not limited to the concept of a horizontal belt filter medium to recover digester liquor from a digester slurry, but can be applied with equal advantageous effect in any system wherein the consistency of a slurry of cellulosic material is increased to form a mat of cellulosic material and the mat then contacted with a treatment liquid, counter currently or otherwise such as, for example, a chemical reactant or a wash liquid, with the objective of having the treatment liquid displace the liquid contained in the mat, with or without a chemical reaction between the treatment liquid and the cellulosic material and/or displaced liquid.
It is, therefore, a principal object of the invention to provide a process for recovering all the cellulosic material which "breaks through" a filtering medium, such as a horizontal belt filter, when a diluted slurry of the cellulosic material is deposited thereon and its consistency increased to form a mat of the cellulosic material.
It is another object of this invention to provide a process wherein said "break-through" cellulosic material is recovered without the need for a separate recovery circuit.
It is another object of this invention to provide a process for defoaming the liquid which passes through a filtering medium, such as a vacuum-operated horizontal belt filter, and recovering the defoamed product.
It is a further object of the invention to provide a process for conveniently establishing or re-establishing equilibrium or steady-state operation during the counter-current contacting of a mat of cellulosic material disposed on a filtering medium, such as a horizontal belt filter, with a treatment liquid which displaces the liquid contained in the mat.
It is another object of this invention to provide an apparatus for reducing loss of vacuum in the operation of a horizontal belt filtering apparatus.
These and other objects of the invention will be apparent to those skilled in the art upon a consideration of this entire specification and the accompanying drawings.